[PATCH v4] mm/huge_memory: preserve PG_has_hwpoisoned if a folio is split to >0 order

[Zi Yan <ziy@nvidia.com>]

folio split clears PG_has_hwpoisoned, but the flag should be preserved in
after-split folios containing pages with PG_hwpoisoned flag if the folio is
split to >0 order folios. Scan all pages in a to-be-split folio to
determine which after-split folios need the flag.

An alternatives is to change PG_has_hwpoisoned to PG_maybe_hwpoisoned to
avoid the scan and set it on all after-split folios, but resulting false
positive has undesirable negative impact. To remove false positive, caller
of folio_test_has_hwpoisoned() and folio_contain_hwpoisoned_page() needs to
do the scan. That might be causing a hassle for current and future callers
and more costly than doing the scan in the split code. More details are
discussed in [1].

This issue can be exposed via:
1. splitting a has_hwpoisoned folio to >0 order from debugfs interface;
2. truncating part of a has_hwpoisoned folio in
   truncate_inode_partial_folio().

And later accesses to a hwpoisoned page could be possible due to the
missing has_hwpoisoned folio flag. This will lead to MCE errors.

Link: https://lore.kernel.org/all/CAHbLzkoOZm0PXxE9qwtF4gKR=cpRXrSrJ9V9Pm2DJexs985q4g@mail.gmail.com/ [1]
Fixes: c010d47f107f ("mm: thp: split huge page to any lower order pages")
Cc: stable@vger.kernel.org
Signed-off-by: Zi Yan <ziy@nvidia.com>
---
From V3[1]:

1. Separated from the original series;
2. Added Fixes tag and cc'd stable;
3. Simplified page_range_has_hwpoisoned();
4. Renamed check_poisoned_pages to handle_hwpoison, made it const, and
   shorten the statement;
5. Removed poisoned_new_folio variable and checked the condition
   directly.

[1] https://lore.kernel.org/all/20251022033531.389351-2-ziy@nvidia.com/

 mm/huge_memory.c | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index fc65ec3393d2..5215bb6aecfc 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3455,6 +3455,14 @@ bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
 					caller_pins;
 }
 
+static bool page_range_has_hwpoisoned(struct page *page, long nr_pages)
+{
+	for (; nr_pages; page++, nr_pages--)
+		if (PageHWPoison(page))
+			return true;
+	return false;
+}
+
 /*
  * It splits @folio into @new_order folios and copies the @folio metadata to
  * all the resulting folios.
@@ -3462,17 +3470,24 @@ bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
 static void __split_folio_to_order(struct folio *folio, int old_order,
 		int new_order)
 {
+	/* Scan poisoned pages when split a poisoned folio to large folios */
+	const bool handle_hwpoison = folio_test_has_hwpoisoned(folio) && new_order;
 	long new_nr_pages = 1 << new_order;
 	long nr_pages = 1 << old_order;
 	long i;
 
+	folio_clear_has_hwpoisoned(folio);
+
+	/* Check first new_nr_pages since the loop below skips them */
+	if (handle_hwpoison &&
+	    page_range_has_hwpoisoned(folio_page(folio, 0), new_nr_pages))
+		folio_set_has_hwpoisoned(folio);
 	/*
 	 * Skip the first new_nr_pages, since the new folio from them have all
 	 * the flags from the original folio.
 	 */
 	for (i = new_nr_pages; i < nr_pages; i += new_nr_pages) {
 		struct page *new_head = &folio->page + i;
-
 		/*
 		 * Careful: new_folio is not a "real" folio before we cleared PageTail.
 		 * Don't pass it around before clear_compound_head().
@@ -3514,6 +3529,10 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
 				 (1L << PG_dirty) |
 				 LRU_GEN_MASK | LRU_REFS_MASK));
 
+		if (handle_hwpoison &&
+		    page_range_has_hwpoisoned(new_head, new_nr_pages))
+			folio_set_has_hwpoisoned(new_folio);
+
 		new_folio->mapping = folio->mapping;
 		new_folio->index = folio->index + i;
 
@@ -3600,8 +3619,6 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
 	int start_order = uniform_split ? new_order : old_order - 1;
 	int split_order;
 
-	folio_clear_has_hwpoisoned(folio);
-
 	/*
 	 * split to new_order one order at a time. For uniform split,
 	 * folio is split to new_order directly.
-- 
2.51.0